1. Field of Invention
The present invention relates to a complementary metal-oxide-semiconductor (CMOS) sensor structure and method of manufacture. More particularly, the present invention relates to a method of manufacturing a CMOS sensor having a silicon-on-insulator (SOI) substrate formed by a nitrogen implant.
2. Description of Related Art
A charge-coupled device (CCD) is a commonly used digital sensing device for extracting images. Applications of CCD are many and have been used in security systems, image recorders and video cameras. However, CCDs are generally expensive and difficult to miniaturize. Due to the current trend for miniaturization, low power consumption and low cost of production, a type of CMOS photo diode device has been recently developed. Employing the latest semiconductor manufacturing techniques to fabricate the CMOS photo diode device. cost of production is low. Moreover, the diode consumes very little energy and is very compact. At present, these CMOS photo diodes are used in PC cameras, digital cameras and some other photographic instruments.
A photo diode is a photosensitive semiconductor device (or light sensing device) that makes use of a P-N junction to convert photonic energy into electrical signals. Due to the presence of an electric field at the P-N junction, electrons in the N-type layer and holes in the P-type layer cannot diffuse across the junction in the absence of light. However, when light of sufficient energy is shone on the junction, molecules near the junction are be activated to generate electron-hole pairs. These electrons and holes then diffuse towards the junction.
In the presence of an internal electric field, the electrons move across the junction into the N-type region while the holes also move across the junction into the P-type region. Consequently, a current is able to flow through the P-N junction. Ideally, the photo diode is in an open-circuit state when surrounded by total darkness.
FIG. 1 is a cross-sectional view showing the structure of a conventional CMOS sensor. As shown in FIG. 1, the CMOS sensor has a P-type semiconductor substrate 100, a field oxide layer 104, a P-well 110, a gate structure 120, an N-type source/drain region 122, an N-type sensor region 124, a depletion region 126 and a borophosphosilicate glass/nitride-silicide glass dielectric protective layer 134.
Since the depletion region 126 is at the P-N junction, electron-hole pairs are generated when a beam of light 140 falls on this region. Hence, light energy is transformed to a current signal. However, the light beam 140 can activate the molecules in the depletion region 126 in a single pass so that its effective interaction length is rather short.
Consequently, only a very small current signal is generated leading to a low contrast ratio. In other words, sensitivity of the photo diode is small. Moreover, a portion of the light ray may penetrate through the depletion region 126 to reach the substrate region 100. When the light ray is absorbed by the substrate 100, undesirable current may be produced.
In light of the foregoing, there is a need to improve the CMOS sensor structure.